1. Field of the Invention
The present invention relates to a valve timing adjuster, which adjusts opening and closing timing (hereinafter, simply referred to as valve timing) of at least one of an intake valve and an exhaust valve of an internal combustion engine.
2. Description of Related Art
For example, a previously known valve timing adjuster includes a housing and a vane rotor. The housing serves as a first rotatable body and is rotated together with a drive shaft, and the vane rotor serves as a second rotatable body and is rotated together with a driven shaft. In the valve timing adjuster, advancing chambers and retarding chambers are arranged one after another in the rotational direction. Each of the advancing chambers and the retarding chambers is formed between a corresponding one of shoes of the housing and a corresponding one of vanes of the vane rotor. Working fluid is supplied to the advancing chambers or the retarding chambers to drive the driven shaft relative to the drive shaft in an advancing direction or a retarding direction to adjust the valve timing.
In such a valve timing adjuster, as recited in, for example, Japanese Unexamined Patent Publication No. 2006-63835, a variable torque is applied to the driven shaft in response to the rotation of the internal combustion engine. The variable torque is a torque that periodically varies, i.e., changes in an advancing direction for advancing the driven shaft or in a retarding direction for retarding the driven shaft in response to the rotation of the internal combustion engine. Here, the variable torque is caused by, for example, a spring reaction force of each corresponding valve, which is opened and closed by the driven shaft. In the valve timing adjuster, in which the variable torque is transmitted through the driven shaft, the phase of the driven shaft (hereinafter, referred to as an engine phase) relative to the drive shaft is set when the torques applied to the driven shaft are balanced. Besides the above-described variable torque, these torques also include, for example, a rotational torque, which is generated by supply of the fluid to the advancing chambers and the retarding chambers.
When a solenoid spool valve, which is used to control the supply of the fluid to the advancing chambers and the retarding chambers, is controlled in the manner described in Japanese Unexamined Patent Publication No. 2006-63835, the engine phase can be limited within a target phase range to substantially hold the valve timing. However, in such a case, when the variable torque reaches, for example, its peak torque and thereby becomes large, the advancing chambers or the retarding chambers may possibly be compressed to cause outflow of the working fluid from the advancing chambers or the retarding chambers. This may possibly cause fluctuating movement (repeated forward and backward rotation) of the vane rotor relative to the housing. This kind of the fluctuating movement may make it difficult to keep the engine phase within the target phase range and thereby to appropriately adjust the valve timing, which is appropriate for the internal combustion engine. Also, it may cause generation of the hammering sound caused by hitting movement of the vane rotor against the housing. Thus, such fluctuating movement is not desirable.